Assembly systems and assembly line balancing

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MEng6106- Manufacturing Systems Modeling & Performance Analysis

Lecture II - Assembly Systems and Line BalancingDr. Tafesse Gebresenbet AAU, Technology Faculty Mechanical Engineering Department Email

References : Mikel Groover, Automation, production systems and CIM Askin, Standridge, Modelling and analysis of Manufacturing systems

Assembly Systems and Line Balancing Assembly involves the joining together of two or

more separate parts to form a new entity, called a subassembly, an assembly or some similar name. Three major categories of processes used to

accomplish the assembly of the components: 1. Mechanical fastening 2. Joining methods 3. Adhesive bounding


Manufacturing systems modeling & perofrmance analysis (TGS)

Assembly Systems and Line Balancing


Mechanical fastening:A mechanical action to hold the components together. Includes:

Threaded fasteners: Screw, nuts, bolts, etc.


Very common in industry. Allow to be taken apart if necessary. Rivets, crimping, and other methods: he fastener or one of the components is mechanically deformed. Press fits: The two parts are joined together by pressing one into the other. Once fitted, the parts are not easily separated. Snap fits: One or both of the parts elastically deform when pressed together. Commercial hardware such as retainers, C-rings, and snap rings may be used. Manufacturing systems modeling & Sewing and stitching: Used to assemble soft, perofrmance analysis (TGS)

Assembly Systems and Line Balancing

2. Joining methods: Includes welding, brazing, and soldering. Molten metal is used to join two or more components together. Common feature of welding techniques is that fusing and melting occur in the metal parts being joined. In brazing and soldering, only the filler metal becomes molten for joining. The metal components do not melt. Not as strong as welding. 3. Adhesive bonding: Involves the use of an adhesive material to join components. Two types of adhesives: thermoplastic and thermosetting. Thermosetting adhesives are more complicated to apply, but are stronger and capable of withstanding Manufacturing systems modeling &perofrmance analysis (TGS)


Assembly systems The methods used to accomplish assembly processes: 1. Manual single-station assembly: Generally used on a product that is complex and produced in small quantities. One or more workers are required depending on the size of the product. Ex: machine tools, industrial equipment, aircraft, ships, etc. 2. Manual assembly line: Consist of multiple workstations. One or more workers perform a portion of the total assembly work on the product. 3. Automated assembly system: Uses automated methods at the workstations rather than human beings. Manual Assembly Lines Used in high-production situations where the work can be divided into small tasks (work elements) and the tasks assigned to the workstations on the line. By giving each worker a limited set of tasks repeatedly, the worker becomes a specialist in those tasks and perform more quickly. (Division of labor)5 Manufacturing systems modeling & perofrmance analysis (TGS)

Assembly systemsTransfer of Work Between Workstations 1. Non mechanical Lines: Parts are passed from station to station by hand. Problems are: Starving at stations Blocking of stations

As a result, cycle times vary. Buffer stocks are used to overcome. 2. Moving Conveyor Lines: Use a moving conveyor (ex. A moving belt, conveyor, etc.) to move the subassemblies between workstations. The system can be continuous, intermittent (synchronous), or asynchronous. Problems of continuously moving conveyor:Starving Producing incomplete items

In the moving conveyor line, production rate may be controlled by means ofq feed rate. = feed rate = conveyor speed (feet per minute or meters per second) = spacing between parts6 Manufacturing systems modeling & perofrmance analysis (TGS)

Assembly systems

Raw work parts are launched onto the line at regular intervals. The operator has a certain time period during which he/she must begin work before the part flows past the station. This time period is called the tolerance time. = tolerance time = length of the station Model Variations It is highly desirable to assign appropriate amount of work to the stations to equalize the process or assembly times at the workstations. This brings the line modeling & Manufacturing systems balancing problem and the threeperofrmance analysis (TGS)


Assembly systems 1. Single Model Line: Specialized line dedicated to the production of a single product. 2. Batch-model Line (Multiple parallel lines): Used for the production of two or more models with similar sequence of processing or assembly operations. 3. Mixed-model Line: Several models are intermixed on the line and are processed simultaneously. These cases may be applied to both manual flow lines and automated flow lines. Type 2 and 3 are easier to apply to manual flow-lines. The problem of line balancing becomes more complicated when going from type 1 to type 3. Manufacturing systems modeling &perofrmance analysis (TGS)


Multiple parallel lines ADVANTAGES easy work load balancing increasing scheduling DISSADVANTAGES higher setup costs higher equipment costs

flexibility higher skill requirements job enrichment slower learning higher line availability complex supervision more accountability As with most problems, multiple objectives exist. By far the most commonly used objective for analytical models is minimization of idle time. However, in practice, real world issues of minimizing tooling investment, minimizing the maximum lift or strain by any worker, grouping tasks requiring similar skills, minimizing movement of existing equipment, and meeting production targets cannot be overlooked.9 Manufacturing systems modeling & perofrmance analysis (TGS)

Workstation cycle time PACED LINES Each work station is given exactly the same amount of time to operate on each unit of product. At the conclusion of this cycle time TC, the handling system automatically indexes each unit to the next station ROLE OF BUFFERS Usually small buffers may be needed in non-automatic assembly to avoid starving. Without buffers if task times vary, un paced (asynchronous) lines may be preferable. UNPACED LINES (ASYNCHRONOUS) The station removes a new unit from the handling system as soon as it has completed the previous unit, performs the required tasks, and then forwards the unit on to the next station. PARALLEL WORKSTATIONS IN SERIAL SYSTEMS In many serial systems, each station along the line is usually a10 Manufacturing systems modeling & perofrmance analysis (TGS)

The Line Balancing ProblemsIt is to arrange the individual processing and assembly tasks at the workstations so that the total time required at each station is approximately the same. Very difficult to achieve perfect balance in most practical situations. If workstation times are unequal, the slowest station determines the overall production rate of the line. TERMINOLOGY Minimum Rational Work Element. The smallest practical indivisible tasks into which the job can be divided. = Time required to carry out this rational work element. Considered to be constant. In fact it varies in a manual station. Assumed that they are additive. In fact it changes when two are Manufacturing systems modeling &perofrmance analysis (TGS)


Total Work Content. ( ) Sum of the time of all the work elements to be done on the line. = Number of work elements that make up the total work or job. Workstation Process Time. ( ) The sum of the times of the work elements done at the station. n= number of stations Cycle Time. ( ) Ideal or theoretical cycle time of the flow line. The time interval between parts coming off the line.12 Manufacturing systems modeling & perofrmance analysis (TGS)

Balance Delay. (balancing loss) (d) Measure of the line inefficiency.

The balance delay d will be zero for any values n and that satisfies the relationship Minimum number of workstations required to optimize the balance delay for a specified may be found by


Manufacturing systems modeling & perofrmance analysis (TGS)

Line Balancing

The Basic objective of Line Balancing problem To assign work elements to workstations such that assembly cost is minimized Total assembly cost includes: Labor cost (while performing tasks) Idle time cost Focus: minimize idle time Limits: production constraints


Manufacturing systems modeling & perofrmance analysis (TGS)

Line BalancingProblem formulation production rate P (units/time) number of parallel lines m number of tasks N time to perform task i : ti total task time T = i=1N ti to meet demand: cycle time Tc =m/p no worker must be assigned a set of tasks of duration longer

than m/p =Tc

Some Features of the Task order partially determined assembly order constraints IP =(u,v) (i.e. task u mustprecede task v)


zoning restrictions task pairs to same station ZS taskManufacturing systems modelingin same workstation ZD pairs not performed &perofrmance analysis (TGS)

Line Balancing

Objective function features lowered number stations fill up first only stations with at least one task are constructed benchmarking gage: proportion of idle time idle time = (paid -productive)

BALANCE DELAY (measures proportion of idle time) D = (K* Tc - ti)/(K* Tc)= idle time/paid time where K* is the number of stations required by the solution16 Manufacturing systems modeling & perofrmance analysis (TGS)

Line Balancing Decision variables task i assigned to station k ? tota